This invention relates to a cooling system for a test head for a semiconductor tester.
A known form of test head for a semiconductor integrated circuit tester is generally parallelepipedal in shape and has two opposite main faces which are nearly square and are spaced at a distance which is considerably less than the length of the shorter sides of the main faces. In the so-called DUT down orientation of the test head, the two major faces are horizontal. For the sake of convenience in the following description of the known test head, it will be assumed, except where the context indicates otherwise, that the test head is in the DUT down orientation.
The known form of test head comprises a housing and multiple pin cards which are located in the housing and are disposed vertically, extending radially from a vertical axis. The pin cards are equidistant from the vertical axis and are spaced therefrom, so that the inner vertical edges of the pin cards surround a vertical circular passage in the test head. Each card has a lower edge at which it is provided with contact pins for engaging a load board, which is attached to the housing and provides an electrical interface between the pin cards and a device under test (DUT). Each pin card carries numerous integrated circuits, including drivers and comparators which can be operated selectively for applying stimulus signals to, or measuring response signals from, the DUT. Operation of the drivers and comparators dissipates substantial heat. In order to prevent overheating of the test head, which can impair the accuracy of a test, exhaust fans are attached to vertical side walls of the housing for exhausting air from the housing through vent openings in the vertical side walls. When the fans are in operation, they create a negative pressure in the housing and draw air into the housing through an air inlet opening in a horizontal top wall of the housing. The air flows at high speed between the cards, where heat is transferred convectively from the electronic components to the air, and is exhausted from the housing through the vertical side walls. In a practical implementation of this form of test head, there are four vent openings, each with an exhaust fan, in each of the four vertical side walls of the housing.
It has been found that this type of cooling system is subject to disadvantage. In particular, it may be desired to use the test head in a laminar flow tent, in which the test head is positioned in a space bounded by curtains and a flow of clean air is induced downward past the test head. It is desirable that the downward flow of clean air should remain laminar, but the flow of air from the fans disrupts the laminar flow. It is further desirable that the clean air should remain at about ambient temperature in order to avoid overheating of the DUT, but the clean air is warmed by the exhaust air from the fans. For example, a test carried out at a room temperature of 23.degree. C. showed temperatures in the range from about 63.degree. C. to 93.degree. C. in the vicinity of the drivers and comparators. In this test, the temperature of the DUT was measured at 51.degree. C. and the temperature of air in the laminar flow tent was 35.degree. C.
In another installation, the test head is used in conjunction with a wafer prober, in which the test head is positioned in a DUT up orientation beneath a device handler which is used for delivering devices to the test head for testing. In this installation, supply of cooling air to the test head is limited by the structure of the wafer prober and the test head cooling system might not be able to maintain the test head at the proper temperature.
In any orientation of the test head, there is a tendency for heated, exhaust air to recirculate back into the test head through the air inlet. This is especially true when the test head is used in the confined space of certain device handlers. This recirculation, or feeding on its own exhaust, results in operation specification drift and yield reduction for tested devices.
The tendency for exhaust air to be recirculated back into the test head is stronger when the test head is in the DUT down orientation than when it is in the DUT up orientation, because in the DUT down orientation the air inlet is above the vent openings. This difference in flow conditions, depending on test head orientation, results in the temperature in the test head being dependent on test head orientation.
Moreover, regardless of the difficulties with respect to use in a laminar flow tent or in conjunction with a wafer prober, or in connection with orientation of the test head, the air flow induced by the sixteen fans in the conventional test head is not optimum for removing heat from the pin cards. Thus, most of the heat dissipated in operation of the test head is dissipated by the drivers and comparators, which are near the bottom of the pin card, close to the contact pins of the pin card, but the major part of the air flow induced by the fans is over the upper regions of the pin cards; therefore, the most heat sensitive components are not impacted directly by the high speed stream of cooling air with consequent reduced reliability, performance and life expectancy.
Further, since there are four fans at each of the four vertical side walls of the test head, the level of noise is substantially the same in all directions and in some applications, the noise generated by the fans may be objectionable to workers in the vicinity of the test head.